1414 J. Org. Chem., Vol. $7, No. 9, 1972
ONOPCHENKO, SCHULZ, AND SEEKIRCHER
Table VII, which lists compounds containing a coplanar zigzag chain, shows that the S value of the OH3a radical is not constant but depends on the kind of Y-20 group. On the other hand, Table VIII, which lists compounds containing a noncoplanar chain, shows that the S value of the OH-30 radical is constant and in-
3a
3cc
5cc Figure 14.-Steroid
7a: skeleton.
dependent of the kind of Y-2a group. One concludes that, although the direct distance between the Y-2 group and the OH-3 radical is much larger in the coplanar zigzag chain, i.e., in the trans conformation, than in the noncoplanar structure, i.e., in the gauche conformation, the influence of the Y-2 group is easily transmitted to the OH-3 radical only through a coplanar zigzag chain.
Conclusion Several theories (or calculation methods) have been presented for estimating the optical rotation of an organic compound.32 Some of these emphasize the role of the atomic refraction, RD (or polarizability, a),of an atom (or a radical) in producing the optical rotatory power of a molecule. However, none of them pay any attention to the variability of or the nature of the change in this RD (or a) value. Moreover, since the days of van’t H ~ f fthe , ~role ~ apparently played by the presence or absence of a coplanar zigzag chain has never been noticed. It is hoped that the empirical rules 1-8 presented in this article will be of help in constructing a new theory or developing new approaches to the calculation of optical rotatory power. Addendum.-The fact that a straight [ R / I l Z o ~vs. S line changes its slope, according to the change in the kind of a Y atom (or radical) which is coplanar with the X atom (or radical) in the same molecule, means that these two atoms (or radicals) (i.e., X and Y) couple with each other to produce a certain partial molecular rotation, in spite of their existence in one plane (cf. 7”, 9’, and 10’; 11, 14,16,and 17; ll’, 16”, and 17”, etc.). (32) J. H. Brewster, in “Topics in Stereochemistry,‘’ Vol. 2, N . L. Allinger and E . L. Eliel, Ed., Interscience, New York. N. Y., 1967, p 1. (33) van’t Hoff, Bu11. SOC.Chim. Fr., 2S, 298 (1875).
Nonclassical Oxidation of Aromatics. I. Cobaltic Ion Catalyzed Oxidations of p-Cymene, p-Ethyltoluene, and sec-Butyltoluenesl ANATOLI ONOPCHENKO, JOHANN G. D. SCHULZ,* AND RICHARD SEEKIRCHER Gulf Research & Development Company, Pittsburgh, Pennsylvania 16650 Received September 61, 1971 Co(II1) ion catalyzed oxidation of alkyltoluenes with oxygen was studied under mild conditions. It was found that the methyl group was preferentially oxidized in the presence of other groups on the same benzene ring. p-Cymene afforded p-isopropylbenzoic acid (90%) and p-methylacetophenone (10%) as the primary products. p-Ethyltoluene gave p-ethylbenzoic acid (68%) and p-methylacetophenone (25%) as major products. Prolonged oxidation converted p-methylacetophenone into p-acetobenzoic acid and eventually into terephthalic acid. A mixture of the isomeric sec-butyltoluenes was oxidized to the corresponding sec-butylbenzoic acids (89%). The relative ease of oxidation of the alkyl groups follows the sequence methyl > ethyl > isopropyl see-butyl. These results cannot be rationalized on the basis of the classical free-radical mechanism, and an elect,ron-transfer mechanism involving the intermediacy of radical cations is proposed.
-
The literature has numerous references to the oxidation of various alkyltoluenes in which secondary or tertiary hydrogen on alkyl groups is preferentially abstracted. I n fact, the oxidation of the methyl group in such instances is difficult. Products are comprised of carboxylic acids, hydroperoxides, and/or cleavage products of the latter. Oxidation of p-cymene or pethyltoluene with 15% nitric acid is reported to produce p-toluic acid, whereas p-ethylisopropylbenzene affords p-ethylbenzoic acid.2 I n the autoxidation of p-ethyltoluene3 or p-~ymene,~--? major products were either (1) A preliminary report by A. Onopohenko, J. G. D. Schultz, and R. Seekiroher appeared in Chem. Commun., 939 (1971). (2) L. N. Ferguson and A. I. Wims, J . Org. Chem., 26, 668 (1960). (3) A. Kobayashi, K. Sadakata, and S. Akiyoshi, Kogyo Kagaku Zasshi, 19, 654 (1956); Chem. Abstr., 62, 5334 (1858). (4) V. B. Fal’kovskii and L. A. Golubko, Ne/tekhzmiya, 8 (3),392 (1968). (5) U. S. Patent 2,833,816 (1958). (6) U. S. P a t e n t 3.227.752 (1966). (7) M. I. Khmura, B. V. Suvorov, and S. R. Rafikov, Zh. Obshch. Khim., as, 1418 (1956).
p-toluic acid, terephthalic acid, p-methylacetophenone, or a combination of these. p-Acetobenzoic acid, p-isopropylbenzoic acid, and p-(a-hydroxyisopropyl)benzoic acid were also formed in varying amounts. The relative ease of oxidation of these alkyl groups follows the sequence isopropyl > ethyl > methyl. I n this paper, we report on a study of Co(II1) ion catalyzed oxidation of a methyl group in preference t o isopropyl, ethyl, or sec-butyl in p-cymene, p-ethyltoluene, and sec-butyltoluenes. Results obtained are surprising as the normal order of hydrogen abstraction, tertiary > secondary > primary, is reversed. Results Reactants, experimental conditions, and the results obtained are summarized in Table I . Conversion in all experiments was essentially 100%. Oxidation of p-cymene gave p-isopropylbenzoic
J. Org. Chem., Vol. 37, No. 9, 1$72
NONCLASSICAL OXIDATION OF AROMATICS
1415
TABLEI OXIDATION OF ALKYLTOLUENES~ Expt no. Hydrocarbon oxidized
Reactants, g Co(0Ac )2 * 4H.10
MEK HOAc
n-CaHlo
1
2b p-Cymene
20 20 400 60 20
20 20 400 85 60
20 20 400 70 53
3 p-Ethyltoluene
4
sec-B utyltoluenesC
20 20 400 100 62
Substrate Conditions Induction time, hr 1.0 0 . 5 min 0.7 0.5 Reaction time, hr 1.5 1.5 0.7 1.5 Products, g (yo) p-Isopropylbenzoic acid 58.5 (90) 10.6 (19) p-Acetobenzoiic acid 6.5 (10) 4.0(7.3) 0.7 (2) p-Ethylbenzoiic acid 16 (68) sec-Butylbenzoic acids 66.0 (89)d p-Toluic acid 9 . 3 (17) 1 . 3 (5.5) p-Methylacetophenone 30 (55) 5.5 (25) Terephthalic acid 1 . 0 (1.8) Trace 36.5% para, 59.0% meta, and 4.5% 105', 22 atm total pressure (partial pressures of butane and oxygen). 20 g of LiCl added. ortho. 38.3% para, 59.5% meta, and 2.2% ortho. Q
acid and p-acetobenzoic acid in yields of 90 and lo%, respectively. To determine whether p-acetobenzoic acid was formed in competition with p-isopropylbenzoic acid, or as the result of secondary oxidation of the latter, one experiment was interrupted after 30 min. p-Isopropylbenzoic acid was shown in this case to be the only acid formed along with p-methylacetophenone. Formation of ketone is evidence for the competitive nature of the reaction. Prolonged oxidation of p-cymene for 5.5 hr resulted in significant amounts of terephthalic acid. Oxidation of p-ethyltoluene under similar conditions gave p-ethylbenzoic acid and p-methylacetophenone in yields of 68 and 25y0, respectively. A mixture of sec-butyltoluenes (36.5% para, 59.0% meta, and 4.5y0 ortho) gave an 89% yield of the corresponding sec-butylbenzoic acids. These results demonstrate unique and unexpected selectivity in methyl group attack in preference over other types of alkyl groups on the same benzene ring.
Discussion The system discussed involves two basic reactions: (a) continuous conversion of Co(I1) ions to Co(III), and (b) interaction of Co(II1) with the aromatic substrates in the presence of oxygen. The first proceeds in the presence of methyl ethyl ketone (MEK)g, or other promoters. As the ketone is rapidly consumed, n-butane was generally used as its precursor and a promoter of greater permanency to ensure complete conversions. Co(I1) ions formed in the course of the second reaction are regenerated to Co(II1) in the first. If neither ketone nor butane is added, Co(II1) ions can be formed by interaction of Co(I1) with peroxy radicals derived from the substrate alone.8-12 PhCHTOO.
+ Co(I1) +PhCHO + Co(II1) + OH-
(8) T. Morimoto and Y . Ogata, J . Chem. SOC.B , 62 (1967). (9) D. A. S. Ravens, T r a n s . Faraday Soc., 66, 1768 (1959). (10) I
